This invention relates to ink jet printing devices and more particularly to a drive transistor for a high resolution thermal ink jet printhead which is integrally formed on a silicon substrate of the printhead containing both the heating elements and printhead addressing circuitry means.
Thermal ink jet printing is accomplished by a droplet-on-demand type printer having a printhead with an array of nozzles from which droplets are selectively ejected. It was early recognized that it was not practical to use one lead for each droplet-ejecting heating element associated with each printhead nozzle. Therefore, active integration of electronic circuitry on the heating-element-containing substrate of a printhead to reduce lead count was implemented rapidly by the ink jet industry.
There is a continual demand for higher resolution printers, meaning more nozzles per inch, because adjacent printed spots are printed by separate nozzles. As the number of nozzles per inch increases well above 300 spots per inch (spi), it was found difficult, if not impractical, to layout compact MOSFET drive transistor switches that fit into the space available behind each heating element without increasing the size of the silicon substrate required. The MOSFET layout that produces the highest current carrying capability alternates transistor sources and drains in parallel arrays behind the heater elements. Depending on the resolution, one or more repeats of a drive transistor may reside behind the associated heater element. When the silicon area required for each printhead increases, the number of printheads which can be made from each silicon wafer decreases, thus driving up the manufacturing cost.
Unfortunately, as the nozzles are moved closer together, the heating elements are likewise moved closer together and the space available to separate drive transistor sources and drains decreases. When attempting to use the smaller space, so as not to increase the required silicon area, the distance from source to drain of the transistors is decreased, causing the depletion region associated with the positively biased drain to extend toward the source and create a subsurface conduction path even when the transistor is in the off state. When this phenomena occurs, the transistor's gate can no longer control conduction through the power MOSFET. This effect is commonly referred to as `punch through` and must be avoided.
Prior art printheads using MOS circuitry are disclosed in U.S. Pat. No. 4,947,192; U.S. Pat. No. 5,010,355; and U.S. Pat. No. 5,159,353, and U.S. Pat. No. 4,308,549; discloses a high voltage field effect transistor.
U.S. Pat. No. 4,308,549 discloses a circular high voltage field effect transistor and process for making it. The transistor has a central drain and concentric annular field plate, gate, and source. Implantation and diffusion techniques are used to produce the source and channel regions. Device dimensions are varied to improve either current, voltage capability, or speed.
U.S. Pat. No. 4,947,192 discloses a printhead formed by monolithic integration of MOS transistor switches on the same silicon substrate containing the resistive heating elements. The transistor switches and heating elements are formed from a single layer of polysilicon with the heating elements formed on a thermally grown field oxide layer having a thickness ranging from about one to four microns.
U.S. Pat. No. 5,010,355 discloses a thermal ink jet printhead having multi-layered ionic passivation of the MOS electronic circuitry which is exposed to the ink. The multi-layered passivation consists of two or three thin film layers to protect the MOS circuitry from mobile ions in the ink. Typical monolithic silicon integrated MOS field effect transistors are described for selective addressing of the heating elements, and these type of monolithic devices are especially susceptible to mobile ions commonly found in inks used by thermal ink jet printers.
U.S. Pat. No. 5,159,353 discloses a thermal ink jet printhead having MOSFET drive transistors which are integrated into the printhead structure. The transistor uses a reduced number of manufacturing steps by utilizing the initial silicon dioxide layer and overlying silicon nitride layer on the silicon wafer, when patterned, as the gate oxide layer in the completed MOSFET transistor. The silicon nitride layer is first patterned for use as a mask to produce the field oxide regions, and then later etched to form the gate.
The present invention solves the problem of enabling high resolution printheads to use reduced silicon substrate areas for the transistors without reducing the high breakdown voltages or requiring that the addressing circuitry be changed.